Recombinant Rat Taste receptor type 2 member 135 (Tas2r135)

What is Tas2r135 and what is its genomic organization?

Tas2r135 (also known as Tas2r28, T2R135, or T2R28) is a G-protein coupled receptor that belongs to the bitter taste receptor family. In rodents, the Tas2r135 gene clusters with Tas2r143 and Tas2r126 on chromosome 6, suggesting they share common regulatory elements and possibly evolved through gene duplication events . These receptors exhibit overlapping histone marks and DNase I hypersensitive sites upstream of Tas2r143, indicating they share common cis-regulatory regions that coordinate their expression patterns . The full-length rat Tas2r135 protein consists of 321 amino acids and functions as a transmembrane receptor involved in bitter taste perception and potentially other physiological processes .

What expression patterns does Tas2r135 show in tissues beyond the gustatory system?

Tas2r135 demonstrates a complex expression profile across multiple non-gustatory tissues. Real-time qPCR analysis has detected Tas2r135 expression in murine vascular smooth cells and high-fat-diet induced mouse fat pads . The Tas2r143/Tas2r135/Tas2r126 cluster is notably expressed in rodent hearts and upregulated under starvation conditions . Through transgenic mouse models using BAC-based CreERT2 systems under the control of the Tas2r143 promoter, researchers have been able to monitor expression patterns of this receptor cluster across various tissues . The presence of Tas2r genes along the gastrointestinal tract varies significantly, with different regions expressing distinct subsets of receptors, suggesting specialized functions throughout the digestive system .

How do expression systems affect the functionality of recombinant Tas2r135?

The choice of expression system significantly impacts the functional properties of recombinant Tas2r135 protein. E. coli expression systems offer high yield and cost-effectiveness but may lack proper post-translational modifications essential for receptor functionality . For functional studies, mammalian expression systems such as HEK-293 cells are generally preferred as they provide an environment that better supports proper protein folding and modifications necessary for signal transduction .

When designing experiments, researchers should consider that functional studies involving bitter taste reception and signal transduction generally require proteins with native-like conformation and appropriate membrane insertion, making mammalian expression systems more suitable despite their higher cost and complexity .

What methodological approaches are effective for studying Tas2r135 ligand interactions?

Several methodological approaches can be employed to study Tas2r135 ligand interactions:

• Calcium Imaging: This technique leverages the fact that bitter taste receptor activation typically triggers calcium release. By co-expressing Tas2r135 with appropriate G-proteins and calcium-sensitive fluorescent reporters in heterologous systems, researchers can monitor receptor activation in real-time .

• BRET/FRET Assays: Bioluminescence/Fluorescence Resonance Energy Transfer assays allow for the detection of conformational changes in the receptor upon ligand binding, providing insights into receptor activation kinetics.

• Electrophysiological Methods: Whole-cell patch-clamp recordings can be used to measure changes in membrane potential or ionic currents following Tas2r135 activation, particularly useful when studying Tas2r135 in native cellular contexts.

• Surface Plasmon Resonance (SPR): This technique enables direct measurement of binding kinetics between purified recombinant Tas2r135 and potential ligands, offering quantitative binding parameters.

For optimal results, a combination of these methods is recommended, as each provides complementary information about receptor-ligand interactions and downstream signaling events .

What genetic approaches can be used to study Tas2r135 function in vivo?

Several genetic approaches have proven valuable for studying Tas2r135 function in vivo:

• CRISPR/Cas9 Gene Editing: This technique has been successfully employed to delete the entire Tas2r143/Tas2r135/Tas2r126 cluster in mice, creating knockout models that allow researchers to assess the physiological roles of these receptors . Such models have revealed surprising findings, such as the observation that these receptors may not be required for bitter tastant-induced bronchodilation, contrary to previous hypotheses .

• BAC-Based Transgenic Models: Bacterial artificial chromosome (BAC) transgenic approaches allow for the expression of reporter genes (such as CreERT2) under the control of the Tas2r143 promoter. When crossed with appropriate reporter lines (e.g., Rosa26-EGFP), these models enable visualization of Tas2r expression patterns across multiple tissues .

• Conditional Knockout Strategies: Tissue-specific deletion of Tas2r135 using Cre-loxP systems provides more refined insights into receptor function in specific cell types or organs, minimizing developmental compensations that might occur in global knockout models.

• Knock-in Reporter Systems: Direct insertion of fluorescent reporters into the Tas2r135 locus enables visualization of endogenous expression patterns while maintaining regulatory elements, offering advantages over transgenic approaches that might not capture all regulatory nuances .

The choice of genetic approach should be guided by specific research questions, with considerations for temporal and spatial control of gene expression or deletion .

How should researchers design experiments to investigate Tas2r135 function in extraoral tissues?

When investigating Tas2r135 function in extraoral tissues, researchers should consider the following experimental design principles:

• Validation of Expression: Before functional studies, confirm Tas2r135 expression in the tissue of interest using multiple complementary methods such as qRT-PCR, immunohistochemistry, and reporter mouse models . The use of multiple detection methods helps overcome potential antibody specificity issues common with GPCR research.

• Cell-Type Specificity: Determine which specific cell types within the tissue express Tas2r135. Single-cell RNA sequencing or fluorescence-activated cell sorting (FACS) of reporter-expressing cells can provide this resolution .

• Physiological Readouts: Select appropriate physiological endpoints relevant to the tissue being studied. For example:

  • In cardiac tissue: contractility, electrical activity, or calcium handling

  • In vascular smooth muscle: vasodilation/constriction responses

  • In gastrointestinal tissues: hormone secretion, motility, or barrier function

• Pharmacological Approaches: Include both gain-of-function (agonist) and loss-of-function (antagonist) studies, alongside genetic approaches (knockout models) to comprehensively assess receptor function .

• Controls for Receptor Specificity: Given the sequence similarity among Tas2r family members, include specificity controls to ensure observed effects are attributable to Tas2r135 rather than related receptors. This might involve comparing responses in wild-type versus Tas2r135-specific knockout models .

A well-designed study would incorporate tissue-specific, cell-type-specific, and receptor-specific controls to rigorously establish Tas2r135 function in the extraoral tissue of interest .

What are the critical factors for successful reconstitution of recombinant Tas2r135 protein?

Successful reconstitution of recombinant Tas2r135 protein requires careful attention to several critical factors:

• Buffer Composition: Optimal reconstitution typically requires Tris/PBS-based buffers (pH 8.0) with stabilizing agents such as trehalose (approximately 6%) . These components help maintain protein stability during the reconstitution process.

• Reconstitution Procedure:

  • Centrifuge the lyophilized protein vial briefly before opening

  • Reconstitute in deionized sterile water to a concentration of 0.1-1.0 mg/mL

  • Add glycerol to a final concentration of 5-50% (typically 50%) for long-term storage

  • Aliquot the reconstituted protein to avoid repeated freeze-thaw cycles

• Storage Conditions:

  • For long-term storage: -20°C to -80°C in small aliquots

  • For working solutions: 4°C for up to one week

  • Avoid repeated freeze-thaw cycles as they significantly reduce protein activity

• Verification of Integrity: Following reconstitution, verify protein integrity through SDS-PAGE and functional assays before proceeding with experiments. Purity of >90% is generally recommended for functional studies .

• Membrane Incorporation (for functional studies): For studies requiring functional receptor, consider incorporating the recombinant protein into appropriate lipid environments (liposomes or nanodiscs) that mimic the native membrane environment of GPCRs .

Adherence to these guidelines maximizes the likelihood of maintaining receptor functionality for subsequent experimental applications.

How can researchers address potential technical challenges in Tas2r135 expression analysis?

Researchers frequently encounter technical challenges when analyzing Tas2r135 expression. Here are strategies to address common issues:

• Low Expression Levels: Tas2r135 often exhibits low expression levels in non-gustatory tissues, making detection challenging. To overcome this:

  • Use highly sensitive methods such as digital droplet PCR or RNAscope for transcript detection

  • Consider enrichment strategies such as laser capture microdissection to isolate specific cell populations

  • Employ nested PCR approaches with proper controls to enhance detection sensitivity

• Antibody Specificity Issues: Commercial antibodies against Tas2r family members often show cross-reactivity. Mitigating strategies include:

  • Validation using knockout tissues as negative controls

  • Use of epitope-tagged recombinant receptors in expression systems

  • Complementing antibody-based detection with genetic reporter systems

• Heterogeneous Tissue Expression: As shown with Tas2r131, expression of bitter taste receptors may be restricted to specific cell types within a tissue (e.g., mucin-producing goblet cells, Paneth cells) . This heterogeneity may mask detection in whole-tissue analyses. Solutions include:

  • Single-cell RNA sequencing approaches

  • Cell-type specific isolation before analysis

  • In situ hybridization techniques with cellular resolution

• Dynamic Regulation: Expression levels may fluctuate with physiological state (e.g., upregulation under starvation) . Design time-course experiments and control for physiological variables that might affect expression.

By anticipating these challenges and implementing appropriate methodological approaches, researchers can more effectively characterize Tas2r135 expression patterns across tissues .

How should researchers interpret findings from Tas2r knockout models compared to pharmacological studies?

When interpreting data from Tas2r knockout models versus pharmacological studies, researchers should consider several important factors that may lead to apparent discrepancies:

A comprehensive approach that integrates both genetic and pharmacological methodologies, alongside careful controls for specificity, provides the most robust framework for data interpretation .

What are the current contradictions in Tas2r135 functional studies and how might they be resolved?

Several contradictions exist in the current literature regarding Tas2r135 function, particularly in extraoral tissues:

• Bronchodilation Mechanism Controversy: While bitter tastants consistently induce bronchodilation, the triple knockout of Tas2r143/Tas2r135/Tas2r126 did not affect this response, suggesting these receptors may not mediate this effect as previously hypothesized . This contradiction might be resolved through:

  • Comprehensive expression profiling of all Tas2r family members in airway tissues

  • Investigation of alternative receptors or direct ion channel effects of bitter compounds

  • Development of more specific pharmacological tools with confirmed receptor selectivity

• Metabolic Function Discrepancies: Some studies suggest bitter taste receptors like Tas2r135 regulate glucose metabolism through GLP-1 release, while other data indicate minimal effects on metabolic parameters . Potential resolution approaches include:

  • Cell-type specific knockout studies focusing on enteroendocrine cells

  • Direct measurement of hormone release in primary cultures from wild-type versus knockout animals

  • Translational studies comparing findings across species (rodent to human)

• Expression Pattern Inconsistencies: Different studies report varying expression patterns of Tas2r135 across tissues . To resolve these discrepancies:

  • Standardize detection methodologies and threshold criteria

  • Account for dynamic regulation under different physiological states

  • Use multiple complementary detection methods (qPCR, in situ hybridization, reporter models)

Addressing these contradictions requires methodological refinement, increased specificity in both genetic and pharmacological approaches, and careful consideration of the complex regulatory mechanisms governing Tas2r135 expression and function across diverse tissues .

How can researchers differentiate between direct Tas2r135-mediated effects and indirect physiological responses?

Differentiating direct Tas2r135-mediated effects from indirect physiological responses is crucial for accurately characterizing receptor function. Researchers should implement the following strategies:

• Temporal Analysis: Direct receptor-mediated effects typically occur rapidly (seconds to minutes) following ligand exposure, while indirect effects may take longer to manifest. Time-course experiments with high temporal resolution can help distinguish these responses .

• Cell-Autonomous Validation: Isolated cell systems expressing Tas2r135 can determine whether observed effects require the cellular context or occur in isolated cells. Approaches include:

  • Primary cell cultures from tissues of interest

  • Heterologous expression systems with reconstituted signaling components

  • Organoid models that maintain tissue architecture while allowing experimental manipulation

• Signaling Pathway Dissection: Tas2r135, as a GPCR, couples to specific G-proteins and downstream signaling pathways. Pharmacological or genetic inhibition of these pathways can determine whether effects are mediated through canonical receptor signaling:

  • G-protein inhibitors (e.g., pertussis toxin for Gαi/o pathways)

  • Calcium chelators to disrupt Ca2+-dependent signaling

  • Specific inhibitors of downstream kinases or second messengers

• Ex Vivo and In Vitro Correlation: Compare responses in intact tissues (ex vivo) with those in isolated cell systems (in vitro) expressing Tas2r135. Concordance between these systems suggests direct receptor-mediated effects .

• Receptor Specificity Controls: Use cells or tissues from Tas2r135 knockout models as negative controls when applying putative receptor ligands. Persistence of effects in knockout tissues strongly suggests indirect or receptor-independent mechanisms .

By systematically applying these approaches, researchers can more confidently attribute observed physiological responses to direct Tas2r135 activation versus indirect or non-specific effects .

What are the emerging areas of Tas2r135 research beyond taste perception?

Several emerging research areas are expanding our understanding of Tas2r135 function beyond traditional taste perception:

• Immunomodulatory Roles: Based on expression patterns in tissues with immune functions, Tas2r135 may participate in immune response regulation. Future research should examine:

  • Tas2r135 expression in specific immune cell populations

  • Effects of receptor activation on cytokine production and inflammatory responses

  • Potential interactions between dietary bitter compounds and immune homeostasis

• Metabolic Regulation: Given the expression of Tas2r family members in enteroendocrine cells and evidence linking bitter taste receptors to glucose homeostasis, future work should address:

  • Cell-specific functions of Tas2r135 in hormone-secreting cells of the gut

  • Impact of Tas2r135 activation on incretin release and insulin sensitivity

  • Metabolic phenotyping of Tas2r135 knockout models under various dietary challenges

• Tissue-Specific Signaling: The same receptor may couple to different downstream pathways depending on the cellular context. Research should investigate:

  • G-protein coupling preferences of Tas2r135 in different tissues

  • Tissue-specific signaling partners and effector molecules

  • Crosstalk with other signaling pathways in non-gustatory tissues

• Therapeutic Targeting: The expression of Tas2r135 in multiple extraoral tissues suggests potential for therapeutic targeting. Future directions include:

  • Development of selective Tas2r135 modulators (agonists/antagonists)

  • Examination of Tas2r135 as a potential drug target in respiratory, digestive, or cardiovascular diseases

  • Investigation of dietary bitter compounds as physiological modulators of Tas2r135-expressing tissues

These emerging areas represent fertile ground for expanding our understanding of this receptor's physiological significance beyond its canonical role in bitter taste perception .

What novel methodological approaches might advance Tas2r135 research?

Several innovative methodological approaches hold promise for advancing Tas2r135 research:

• CRISPR-Based Techniques Beyond Gene Deletion:

  • CRISPR activation (CRISPRa) or CRISPR interference (CRISPRi) for temporal control of Tas2r135 expression

  • CRISPR-based precise knock-in of reporter tags at endogenous loci

  • Base editing to introduce specific mutations that affect receptor function without complete deletion

• Advanced Imaging Approaches:

  • Genetically encoded biosensors that directly report Tas2r135 activation in real-time

  • Super-resolution microscopy to visualize receptor clustering and membrane organization

  • Intravital microscopy to observe receptor dynamics in living tissues

• Proteomics and Interactomics:

  • Proximity labeling techniques (BioID, APEX) to identify Tas2r135 interaction partners in different tissues

  • Quantitative phosphoproteomics to map signaling networks downstream of receptor activation

  • Crosslinking mass spectrometry to characterize receptor complexes

• Computational and Structural Biology:

  • Molecular dynamics simulations of Tas2r135-ligand interactions

  • Homology modeling based on recently solved GPCR structures

  • In silico screening for novel ligands with enhanced selectivity

• Organoid and Microphysiological Systems:

  • Tissue-specific organoids expressing fluorescent Tas2r135 reporters

  • Organ-on-chip models incorporating Tas2r135-expressing cells

  • Co-culture systems to study intercellular communication mediated by Tas2r135 activation

These advanced approaches, particularly when used in combination, can provide unprecedented insights into Tas2r135 biology across multiple physiological contexts .